1. Field of Invention
This invention relates generally to fabrics for sportswear, and more particularly to a composite fabric for use in sportswear intended for skiers and other athletes who are active in a cold environment.
2. Status of Art
The clothing worn by athletes in competitive play in the course of which they undergo strenuous physical activity may adversely affect their performance. Because the fabric of wearing apparel more or less conforms to the body of the wearer and is interposed between the body and the atmosphere, it therefore acts effectively as a heat exchanger or thermal barrier, depending on its inherent characteristics.
The concern of the present invention is with garments for skiers and other athletes who perform in the open air in a cold and possibly windy and wet climate. The primary requirement for such sportswear is that it functions to keep its wearer warm and dry. It is also essential that the garment be light-weight and without undue bulk so as not to interfere with sports activity.
To some degree, these two requirements are incompatible; for one normally obtains warmth with heavily padded clothing, and a garment of this type is unsuitable for a skier or other athlete. The ideal clothing for an athlete whose sport is practiced in a cold environment is one which is light-weight and free of bulk; hence in no way interferes with freedom of movement, and yet thermally protects the wearer and keeps him warm and dry. It is also desireable that the garment be effectively waterproof, so that it protects the wearer when it rains or snows, yet be vapor-permeable so as not to seal in vapors emanating from a perspiring body and thereby give the wearer a clammy feeling.
By way of background to the invention, we shall now consider those factors which cause an athlete to generate body heat and those which give rise to a loss of body heat.
The transfer of heat, whether between a human body or an inanimate body and the surrounding atmosphere, takes place by three distinct processes: conduction, convection and radiation. In conduction, heat is transferred by the short range interaction of molecules and/or electrons. Convection involves the transfer of heat by the combined mechanisms of fluid mixing and conduction. In radiation, electromagnetic energy is emitted toward a body and the energy incident thereto is absorbed by the body to raise its temperature. Radiant heating, therefore, differs from both convection and conduction heating, for the presence of matter is not required for the transmission of radiant energy.
According to the Stefan-Boltzmann law, the rate of heat transfer between a source of radiated heat whose temperature is T.sub.s and an absorbing body whose temperature is T.sub.b is proportional to T.sub.s.sup.4 -T.sub.b.sup.4 ; that is, to the difference between the fourth powers of these temperature values. In convection heating, the rate of heat transfer is proportional only to the temperature difference between the body being heated and the surrounding atmosphere. Hence convection heating is inherently very slow as compared to the nearly instantaneous effects of radiant heating.
The interior of the human body has a normal temperature level which is usually said to be 98.6.degree. F. But actually, in the course of each 24-hour period, the body temperature rises above and falls below this nominal value within a 5.degree. F. range. Body temperature is determined by the relation existing between the amount of heat internally generated, which depends on basal metabolism, and the amount of heat escaping from the body. Additional heat is produced as a result of muscular activity, this being dissipated by an increase in radiation, conduction or evaporation from the skin surface by more rapid and deep breathing. Thus the skin is the interface between the internally heated body and the atmosphere.
In an intensely cold environment in which the ambient temperature is well below zero degrees centigrade, the temperature differential between the ambient temperature and the human body temperature is substantial. This differential results in heat transfer between the body and the environment at a rapid rate. Hence, unless the clothing worn by a skier is such as to interpose an effective thermal barrier between the wearer's body and the atmosphere, the skier may become dangerously cold.
In order to provide sports clothing which acts as an effective thermal barrier and yet is light-weight and free of bulk, it is now known in ski jackets and in other garments intended for use in a cold climate to provide a light-weight outer shell of nylon fabric or Gortex which acts as a waterproof windbreaker, yet is vapor-permeable. Because this shell affords little warmth, the shell is usually lined with a thin layer of non-woven hollow fibers which act to trap air and minimizes convection heat losses. Such hollow fiber layers are available commercially under various well-known trademarks such as "Thinsulate", "Hollofil", and "Qualofil".
While a "Thinsulate" or similar layer which lines the shell of a ski jacket minimizes the loss of heat from the body by convection, the heat absorbed by the "Thinsulate" layer from the body is radiated therefrom, and this radiation which results in a substantial loss of heat is not significantly impeded by the outer shell of the jacket. Hence, while the typical ski jacket which has a "Thinsulate" liner is light-weight and quite warm, even though lacking in bulk, it nevertheless falls well short of a skier's requirements in an intensely cold climate.
The effectiveness of a "Thinsulate" ski jacket is deceptive; for when the wearer is highly active with a resultant increase in the amount of internally-generated body heat, then the jacket is acceptably warm; but when the skier is being conveyed up a slope by a ski lift or is otherwise inactive, the thermal protection then afforded by the jacket may be inadequate.